Along with an increase in the switching speeds of power semiconductor devices such as IGBTs (insulated gate bipolar transistors), the torque control and speed control of an alternating current motor variable-speed drive system including these power semiconductor devices has become better, which makes a large contribution to energy savings in the total system.
On the other hand, electromagnetic interference (EMI) attributable to high-speed switching in a power semiconductor device is becoming a crucial issue in the our highly information-oriented and computerized society. For example, in an inverter-driven alternating current motor, a shaft voltage is generated between the motor shaft and the motor frame. The resultant high-frequency leakage current that flows through parasitic capacitors of the motor generates electric corrosion in the bearings. This can cause problems in the rotation of the motor and has the potential of being a large noise source in the power line Internet, in which PCs communicate with other devices through the power line, which will come into practical use in the future.
As a countermeasure against EMI in the power electronics devices, there has already been proposed, for example, an EMI filter that suppresses a rapid change in the current and voltage by using a reactor or a filter, as described in “A method of measuring EMI generated by an alternating current motor drive system using a PWN inverter, and a method of decreasing the EMI”, by Ogasawara and two other engineers, in the IEEJ (the Institute of Electrical Engineers of Japan) Transactions on Industry Applications, 1996, Vol. 116, No. 12, pp. 1211-1219.
As a countermeasure against the EMI in power electronics devices, there has also been proposed an EMI filter that offsets a common mode voltage of an inverter by using active elements like transistors, as described in “Active EMI Filter for Switching Noise of High Frequency Inverters”, by I. Takahashi and three other engineers, the Proceeding of Power Conversion Conference (PCC)-Nagaoka, 1997, pp. 331-334.
As a countermeasure against the EMI in the power electronics devices, there has also been proposed an EMI passive filter that changes a phase voltage into a sinusoidal wave, as described in “Design and characteristics of a three-phase sinusoidal wave voltage output PEM inverter system-passive EMI filter that does not generate a common mode voltage”, by Hasegawa and two other engineers, in the IEEJ Transactions on Industry Applications, 2002, Vol. 122, No. 8, pp. 845-852. According to this technique, an EMI filter is constructed using only passive elements such as a normal mode choke (what is called an alternating current reactor), a common mode choke (what is called a zero-phase reactor), a resistor, and a capacitor, thereby changing three-phase line-to-neutral voltages and three-phase line-to-line voltages at the ac terminals of the inverter into sinusoidal waves. Therefore, substantially all the shaft voltage can be eliminated.
As a countermeasure against the EMI in the power electronics devices, there has also been described a common mode circuit that grounds the neutral point of a motor and grounds the neutral point at the direct link, in “Motor Shaft Voltage and Bearing Currents and Their Reduction in Multilevel Medium-Voltage PWM Voltage-Source-Inverter Drive Applications”, by Fei Wang, IEEE transaction on Industry Applications, September and October, 2000, Vol. 36, No. 5.
In Japanese Patent Application Unexamined Publication No. 2001-352792, there has been proposed a technique of smoothing a voltage, having relatively large amplitude, of a rectangular wave at the neutral point with a view to decreasing the noise generated by an alternating current motor and preventing rotation problems.
According to the above conventional techniques, there has been provided a certain effect of suppressing the EMI, but at an insufficient level.
Among the above techniques, the EMI filter using a active elements such as transistors requires complementary transistors. Further, due to the limit of a withstand voltage of obtainable transistors, this EMI filter can be applied to only to a 200 volt or less inverter. The EMI also has a complex structure and, accordingly, has a high manufacturing cost.
On the other hand, an EMI passive filter having a normal mode choke and a common mode choke can change the output phase voltage and the line voltage of the inverter into a sinusoidal wave, and therefore, can suppress the shaft voltage. However, because a normal mode choke is necessary, the device becomes large and has a high manufacturing cost.
Therefore, in the light of the above problems, it is an object of the present invention to provide a compact low-price filter device having strong effect on suppressing electromagnetic interference in a filter device that suppresses the electromagnetic interference generated in an alternating current circuit connected to a power converter with an alternating current output.